AS5304/AS5306 Integrated Hall IC for linear and off-axis rotary motion detection

Revision 1.5

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Page 9 of 13

8.1

Resolution and Maximum Rotating Speed

When using the AS5304/AS5306 in an off-axis rotary application, a multi-pole ring magnet must be used. Resolution, diameter

and maximum speed depend on the number of pole pairs on the ring.

8.1.1

Resolution

The angular resolution increases linearly with the number of pole pairs. One pole pair has a resolution (= interpolation factor) of

160 steps or 40 quadrature pulses.

Resolution [steps] = [interpolation factor] x [number of pole pairs]

Resolution [bit] = log (resolution[steps]) / log (2)

Example: multi-pole ring with 22 pole pairs

Resolution

= 160x22 = 3520 steps per revolution

= 40x22 = 880 quadrature pulses / revolution

= 11.78 bits per revolution = 0.1023° per step

8.1.2

Multi-pole Ring Diameter

The length of a pole pair across the median of the multi-pole ring must remain fixed at either 4mm (AS5304) or 2.4mm

(AS5306). Hence, with increasing pole pair count, the diameter increases linearly with the number of pole pairs on the magnetic

ring.

Magnetic ring diameter = [pole length] * [number of pole pairs] / π

for AS5304: d = 4.0mm * number of pole pairs / π

for AS5306: d = 2.4mm * number of pole pairs / π

Example: same as above: multi-pole ring with 22 pole pairs for AS5304

Ring diameter =

4 * 22 / 3.14 = 28.01mm (this number represents the median diameter of the ring, this is where the

Hall elements of the AS5304/AS5306 should be placed; see Figure 4)

For the AS5306, the same ring would have a diameter of: 2.4 * 22 / 3.14 = 16.8mm

8.1.3

Maximum Rotation Speed

The AS5304/AS5306 use a fast interpolation technique allowing an input frequency of 5kHz. This means, it can process

magnetic field changes in the order of 5000 pole pairs per second or 300,000 revolutions per minute. However, since a magnetic

ring consists of more than one pole pair, the above figure must be divided by the number of pole pairs to get the maximum

rotation speed:

Maximum rotation speed = 300,000 rpm / [number of pole pairs]

Example: same as above: multi-pole ring with 22 pole pairs:

Max. speed = 300,000 / 22 = 13,636 rpm (this is independent of the pole length)

8.1.4

Maximum Linear Travelling Speed

For linear motion sensing, a multi-pole strip using equally spaced north and south poles is used. The pole length is again fixed

at 2.0mm for the AS5304 and 1.2mm for the AS5306. As shown in 8.1.3 above, the sensors can process up to 5000 pole pairs

per second, so the maximum travelling speed is:

Maximum linear travelling speed = 5000 * [pole pair length]

Example: linear multi-pole strip:

Max. linear travelling speed = 4mm * 5000 1/sec = 20,000mm/sec = 20m/sec

for AS5304

Max. linear travelling speed = 2.4mm * 5000 1/sec = 12,000mm/sec = 12m/sec

for AS5306